In some countries, and particularly in the United States of America, it is usual at the present time to carry out headlamp beam adjustment using an apparatus which is fitted in a well-defined position with respect to the headlamp, by applying three reference points of the apparatus against three homologous reference bosses or projecting elements which are provided on the external surface of the cover glass of the headlamp, and which are integral with it. The positions of these three bosses are clearly defined and themselves define a plane of reference which may for example be vertical, or which may have predetermined inclination with respect to the vertical plane. For more details of this method of adjusting a headlamp beam, reference is made to the current United States standard FMVSS108.
One major difficulty in the manufacture of cover glasses which have to be provided with such bosses is encountered in the case of headlamps with inclined cover glasses. The inclination of the glass may be required, especially for aerodynamic or styling reasons, and the inclination may be such that the normal vector is directed upwardly (in the case where the cover glass forms a forward extension of a hood having a relatively forwardly plunging profile), and/or in which the normal vector is directed towards the side (in the case where the cover glass is in a rounded corner region of the vehicle).
Reference is here made to FIG. 1 of the accompanying drawings, which shows in vertical axial cross section an inclined cover glass G which is formed with conventional bosses, or projecting elements, B1 and B2. Because of the inclination of the cover glass, it Can be seen that the upper boss B1, in order to define with the boss B2 (and with a third boss not shown) a vertical reference plane PV, must project forward from the general plane of the cover glass by a considerable amount. Such a boss is firstly ugly in appearance, and secondly dangerous, especially in the event of collision between the vehicle and a pedestrian. In addition, such a projecting boss can prove to be rather fragile.
Another disadvantage of the prior art technique lies in the fact that the three standardised bosses, which must by nature be included in the general contour of the glass are spaced apart from each other by quite large amounts. It is therefore not possible to make headlamps in which the cover glass is smaller than the limiting size that enables the three bosses to be circumscribed.